Non-dispersive infrared gas analyzer with unbalanced operation

ABSTRACT

Separate pulsed sample and reference beams of infrared energy are passed, respectively, through sample and reference gases to separate chambers in a detector unit for measuring the absorption of the sample beam by a component of interest in the sample gas. In the detector unit, the relative absorption of the two beams is measured by a flow responsive sensor in a passage connecting the two chambers, the sensor having a rate of change of resistance that varies inversely with temperature. By substantially unbalancing the two beams, in the absence of a component of interest in the sample gas, so that the sample beam is the stronger before the beams enter the detector unit, the sensitivity of the instrument is greatly increased because of the nonlinear response of the sensor. The greater the initial unbalance, the greater the sensitivity. In the extreme case, the unbalance amounts to complete suppression of the reference beam, so that the analyzer becomes a single beam instrument.

United States Patent (72] Inventors Adrian C. Billetdeaux Pittsburgh;John P. Strange, Murrysville, Pa. [21] AppLNo. 766,226 [22] Filed Oct.9, 1968 [45] Patented Feb. 2, 1971 [73] Assignee Mine Safety AppliancesCompany Plttsburgh,Pa. a corporation of Pennsylvania [54] NON-DISPERSIVEINFRARED GAS ANALYZER Primary Examiner-William F. LindquistAnorney-Brown, Critchlow, Flick & Feckham ABSTRACT: Separate pulsedsample and reference beams of infrared energy are passed, respectively,through sample and reference gases to separate chambers in a detectorunit for measuring the absorption of the sample beam by a component ofinterest in the sample gas. In the detector unit, the relativeabsorption of the two beams is measured by a flow responsive sensor in apassage connecting the two chambers, the sensor having a rate of changeof resistance that varies inversely with temperature. By substantiallyunbalancing the two beams, in the absence of a component of interest inthe sample gas, so that the sample beam is the stronger before the beamsenter the detector unit, the sensitivity of the instrument is greatlyincreased because of the nonlinear response of the sensor. The greaterthe initial unbalance, the greater the sensitivity. In the extreme case,the unbalance amounts to complete suppression of the reference beam, sothat the analyzer becomes a single beam instrument.

PATENTEB FEB 2197! 3,560,735

ADRIAN C. BILLETDEAUX JOHN P. STRANGE kmmqmwcum INVENTORS.

\ ATTORNEYS.

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sum- 3 or 3 1NVENTORS. ADRIAN c. BILLETDEAUX JOHN P. STRANGE kATTORNEYS.

general type in which'the gas component-of interest is selectivelydetected by its absorptionof a pulsed beam'of infrared energy and by themeasurementof the resulting attenuation of 7 that beam. It is a primaryobjectof theinvention to provide such an analyzer that incorporates adetector unit having an electrically heated resistance sensor responsiveto cooling by gas flow andhaving a rate of change of resistance thatvaries nonlinearly with temperature, thatincludes means forsubstantiallyunbalancing the infrared energy reaching the detector unit in theabse'nce of thecomponent of interest in a way that will" enhance thefavorable nonlinear characteristics of such a sensor, and that, intheextreme case, permits the operationof the instrument as a single beaminstrument.

Further objects of the invention will be apparent from the followingdescription .of a preferred embodiment in connection with the attacheddrawings, in which:

, FIG. 1 represents a, diagrammatic view of a nondispersive double beaminfrared analyzer with means for attenuating one of those beams toprovide unbalanced operation;

FIG. 2 is an end view of the beam chopper shown in FlG. l;

FIG. 3 is a diagrammatic view of a nondispersive single beam infraredgas analyzer representing the extreme form of unbalanced operation;

FIG. 4 is a graph showing the relationship between infrared absorptionby a component gas of interest and the output signal of the detectorunit for balanced operation and for various degrees of unbalancedoperation;

FIG. 5- is a diagram showing the flow transfer characteristic of thesensor in response to gas flow (input signal) produced by pulsedradiation in the presence and in'the absence of absorption by acomponent of interest in the gas to be analyzed and the resulting changein resistance of the sensor (output signal) in the operation of thesingle beam analyzerof FIG. 3, representing the extreme case ofunbalanced operation in which the reference beam is entirely suppressed,

FIG. 6 is a diagram similar toFlG; 5, showing the operation of theanalyzer of H6. 1, with the two beams balanced in the absence of thecomponent of interest; and

FIG. 7 is a diagram similar to FIG. 5, showing the operation of theanalyzer of FIG. 1, with the two beams unbalanced by 50 percent (i.e.,theinfrared energy in the sample beam is twice that in the referencebeam), inthe absence of the component of interest in the gas sample.

In. the usual pulsed double beam infrared analyzer, one

, beam (the sample beam) passes through the gas sample to be analyzedand the other (the reference beam) passes through a reference gas. Eachbeam thenenters'a separate gas absorption chamber in a detector unitcontaining a gas that absorbs infrared energy. In such instruments, thetwo beams are initially. balanced, or substantially so, when they enterthe detector unit, i.e., they contain substantially, equal amounts ofinfrared energy. If the sample gas contains a component of interest thatabsorbs such energy, the sample beam will be attenua'ted before itreaches the detector unit, and that attenuation can be measured, as forexample, by measuring the differential pressure pulses generated in thetwo chambers. In other words, in conventional operation',1the beams areinitially balanced'and then unbalanced by attenuation of the samplebeamin'response to the component of interest in the sample. If theattenuation is slight, as it is with low absorptions by the component ofinterest, the differential pressure'pulses between the chambersof thedetector unit will be very small and maybe offset by other losses in thesystem or by noise.

The present invention, in contrast, provides for initial unbalancebetween the sample and reference beams, with the sample beam beingsubstantially the stronger, as both beams enter the detector unit in theabsence of the component of interest in the sample gas. This initialunbalance is large enough to maintain an unbalanced condition even atmaximum attenuation'of the sample beam by absorption by the component ofinterest. Preferably, this initial unbalance is suppressed orcompensated for in the measuringcircuit. If the component of interest ispresent in the sample gas, the sample beam will be attenuated as itreaches the detector unit, but it will still be substantially strongerthan the reference beam. The present The unbalanced operation describedabove is predicated on taking advantage of certain physicalcharacteristics of a sonsor, such as a thermister, that has a nonlinearresponse to the cooling effect of gas flow over its surface. Athermister is a thermally sensitive electrically resistive element witha negative temperature coefficient of resistance, i.e., its resistanceincreases with a decrease in its temperature. Further, the rate ofchange of resistance varies inversely with the temperature, i .e., theresistance increases at'a faster rate for each change of unittemperature as the temperature decreases. These characteristics give tothe thermister a nonlinear response to gas flow 'thatis highly favorableto the use of such an element as a sensor responsive to a decrease intemperature. If the thermister is heated by an applied external voltageto a temperature above the ambient temperature and if a gas of lowertempera ture is flowed over the thermister surface, the cooling efiectof the gas will produce an increased electrical resistance in thethermister and the rate of changeof that resistance will in turnincrease the lower the temperature to which the thermister is cooled. Ifthe velocity of the gasflowing over the surface of the thermister isincreased, there results a disproportionate increase in output signal orsensitivity, merely because of the change in gas velocity. In otherwords, if the gas velocity is doubled the increase in response is morethan doubled.

Accordingly, the flow responsive detector unit of the present inventionis provided with a pair of gas chambers at least one of which is adaptedto be irradiated by a pulsed beam of infrared energy and to contain agas absorbing such energy.

A gas flow chamber is connected to each of the gas chambers by aseparate passage that terminates atthe flow chamber in a .jet orifice.These orifices provide high velocity jet, flow in the flow chamber inresponse to the alternate expansion and contraction of the gas in atleast one of the gas chambers when subjected to pulsed radiation. inother words, the gas that is irradiated and contains a radiationabsorbing component will altemately increase and decrease intemperature, thereby causing the gas to expand and contract and to flowfrom one gas the orifices. The response of the sensor can'be used tocreate a fluctuating signal in a conventional electrical measuringcircuit.

The foregoing detector unit is the subject matter of the copendingapplication Ser. No. 766,227, filed Oct. 9, I968 of John P. Strange, oneof the inventors herein, and assigned to the same assignee, and filed ofeven date herewith, for Flow ResponsiveDetector Unit and itsApplications to Infrared Gas Analyzers.

In FIG. 1, the analyzer is of the double beam nondispersive type thatincludes a source of infrared radiation, shown here as two substantiallyidentical, side-by-side sources 1 and 2, although it is obvious thatthey could be combined into a single source and then split, fortransmitting radiation along two parallel beam paths. One of thosebeams, herein called the analytical beam, is composed of rays fromsource 1 extending parallel to the optical axis 3 (shown in brokenlines). The other beam, herein called the reference beam, consists ofrays from source 2 and extends parallel to the axis 4. The analyticalbeam passes from source I through a chopper region, where the rays areperiodically interrupted by a shutter device 5 rotated by an electricmotor 6. The beam then goes through a sample or analytical cell 7, whichis provided with a gas inlet 8 and a gas outlet 9 and is otherwisesealed from the atmosphere by infrared-transparent windows 11 at eachend. Finally, the analytical beam enters an analytical gas absorptionchamber 12 through a window 13 similar to windows 11.

The reference beam follows a path parallel to the analytical beam,passing from source 2 through the chopper region traversed by shutter 5,then through a reference cell 14 similar to the sample cell 7, exceptthat the reference cell contains a fixed volume of reference gas, whichis preferably of the same composition as the sample gas but without theparticular component that is to be detected and measured. After leavingthe reference cell, the reference beam enters a reference gas absorptionchamber 16, which is identical with the analytical gas chamber 12previously described.

The gas chamber 12 and 16 are part of a detector unit 21. Generally,that unit includes a flow chamber 22; a thermister 23 mounted in theflow chamber; flow passages 24 and 26 connecting gas chambers 12 and 16,respectively, to the flow chamber; and jet orifices 28 forming theterminal portions of the passages 24 and 26. In the form of analyzerillustrated in FIG. 1, the detector unit with its constituent chambersand passages may be entirely filled with a mixture of (a) the gaseouscomponent to be detected, or some other infrared absorbing gas, and (b)a nonabsorbing diluent gas.

An adjustable trimmer 31 is provided in the optical path of thereference beam for reducing the energy in that beam before it reachesgas chamber 16 in the detector unit. The trimmer can be inserted more orless into the port of the beam to occlude it by the desired amount.Alternatively, the electrical power to the source 2 can be cut down todecrease the beam energy.

Referring generally to the operation of the analyzer illustrated in FIG.1, the two beams of infrared energy are transmitted along the analyticaland reference paths through the sample and reference cells to the gaschambers of the detector unit. These beams are simultaneously andperiodically interrupted by the rotary chopper 5, so that pulses ofinfrared energy at the chopping frequency pass along the two beam paths.If the pulses reaching the detector unit contain energy in those wavelengths that are absorbed by the gas in that unit, the gas therein willbe heated and tend to expand in accordance with the gas laws. If bothgas chambers 12 and 16 receive and absorb the same amount of energy thegas expansion in each of those chambers will be equal and there will bean increase in pressure therein and in passages 24 and 26 and in flowchamber 22, but there will be no gas flow through the flow chamber. Thiscondition will prevail, for example, when the two beams have the sameenergy (under balanced conditions) and when the sample cell 7 contains asample gas that includes none of the component to be detected and thereference cell 14 contains a similar gas.

In contrast, under unbalanced conditions when the energy in the twobeams is unequal, with that in the sample beam being substantiallygreater than that in the reference beam (as by insertion of the trimmer31 into the optical path of the reference beam or, in the case ofextreme unbalance shown in FIG. 3, where the reference source andreference cell are completely eliminated), there will be greaterinfrared absorption in gas chamber 12 of the detector unit (which isirradiated by the sample beam) than in gas chamber 16 (which isirradiated by an attenuated reference beam or, as in FIG. 3, by no beamat all). As a result, gas in chamber 12 will expand more than in chamber16 and create a pulsing flow of gas through the connecting passages andflow chamber 22.

The pulsating gas flow through the flow chamber 22 will cool thethermister, and the cooling effect can be measured as a change inresistance. As shown in FIG. 1, the thermister is one arm of anelectrical bridge circuit that also includes fixed resistances R1, R2,and R3 in the other arms. A battery 32 supplies current to the bridge.The pulsed output of the bridge is connected to an AC amplifier 33,which is followed by a rectifier 34, and a smoothing or averaging filter36 that includes a capacitor 37 and a resistor 38 in parallel. Theresulting DC output signal, which is proportional to the AC component ofthe resistance change of the thermister, is preferably balanced by abucking circuit 39, so that the zero of the output indicator 41 may bemade to correspond with any level of thermister signal within the rangeof the amplifier system in the absence of the component of interest inthe sample gas. The bucking circuit may take the form of a potentiometer42 and a source of direct current 43 connecting across the indicator 41,as shown in FIG. 1. The indicator 41 may be connected to eitherpolarity, so that an increase or decrease in the magnitude of thethermister AC signal may be considered upscale.

To illustrate what happens under the initially unbalanced conditionsreferred to, there is shown in FIG. 4 a family of analytically derivedcurves of output signal vs. sample (component of interest) absorption,with the ratio of reference source power to sample source power as aparameter. With a reference to sample source ration of 0, which meansthat the reference source is deenergized and the instrument is operatingsingle beam" as in FIG. 3, and with the resulting unbalance outputsignal zeroed in the absence of the component of interest in the sample,the output signal is related to sample absorption as indicated by thetop curve of FIG. 4. If the reference source is made equal to the samplesource, a reference to sample ratio of I (i.e., balanced sample andreference beams), the output signal follows the bottom curve of FIG. 4.In other words, the output signal for any amount of sample absorption isseen to be considerably diminished by balancing" the beams. Intermediateamounts of unbalance produce output signal vs. sample absorptioncharacteristics intermediate between maximum unbalance (reference/sampleO) and perfect balance (reference/sample l FIG. 4 was constructedassuming a parabolic (y= a bx) relationship for the nonlinear responsecharacteristics of the sensor 23 (i.e., increasing positive slope withincrease in variable). Such a curve is shown in FIGS. 5, 6, and 7. Inthe upper left portion of these figures, a nonlinear transfercharacteristic 50 having symmetry about the ordinate is shown. Asapplied to the flow responsive sensor of the detector unit describedherein, the ordinate represents thermister resistance or, with theappropriate circuit, thermister voltage. The abscissa represents flowbetween the gas absorption chambers 12 and 16 of the detector unit. Whenonly one of the gas chambers is irradiated (as in FIG. 3), the resultingincrease in temperature and pressure produces a flow out of chamber 12into the second chamber 16, which ceases when the pressure builds upsufiiciently in the second chamber. When the radiation is shut off, flowwill return from the second chamber to the first chamber as the gas inthe first chamber cools. If the radiation into the one chamber of thedetector is chopped or pulsed, as by the shutter 5, and sufficient timeallowed for temperature equilibrium to be' reached, the total flow inone direction will be equal to the total flow in the other direction.Also, if the equilibrium time is long compared with a cycle of thechopper, the flow waveform will be similar to the radiation waveforms;thus the abscissa of the transfer curve also represents beam energy.

The diagrams in FIGS. 5-7 can be most easily understood by reference toFIG. 5, which shows the chopped, single beam operation, and theresulting output signal of the analyzer shown in FIG. 3. In the lowerleft portion of FIG. 5, a flow or radiation waveform is shown with a netvalue of zero; i.e., equal right and left values on alternate halfcycles of the chopper 5. The solid line 51 represents the flow waveformwith no absorption in the sample (i.e., the component of interest is notpresent). The broken line 52 represents the flow waveform withabsorption in the sample due to the presence of the component ofinterest. In each case, a triangular wave is indicated for convenience.If values are transferred point by point from the abscissa, or flowaxis,to the ordinate, or signal axis, using the transfer'characteristic ofthe curve 50, the output signal may be derived. This is shown in theupper right portion of the f gure with a solid line 54 representing theno sample absorption" signal and the broken line 56 representing thesignal obtained when the beam energy is diminished by absorption in thesample. The absorption signal is then obviously merely the differencebetween the solid line and the broken line, shown by the bar 57.

FIG. 6 represents, for comparison, the situation when both detectorchamberslZ and 16 are irradiated under balanced operating conditions.Here, the-flow waveform 61 resulting from the reference-beamis shownasalways opposite to the flow waveform 62 from the sample beam, sincethose beams are pulsed simultaneously by the shutter 5. When thereferenceand sample beamscontain equal infrared energy, i.e., balancedoperation with no sample absorption, there will be no net flow throughthe flow chamber 22, as indicated by the heavy solid line 63. However,when the sample beam is attenuated by sample absorption, the flowinduced by the sample beam in chamber 12 will be less, as shown by theline 64. The net flow, a pulsing flow, through the flow chamber 22 willbe the algebraic sum of the corresponding abscissae points on lines 61and 64. This result is shown as line 66 in FIG. 6, representing the netpulsating flow through the chamber 22. Again, transferred point bypoint, the output signals may be projected as they were in FIG. 5, andthe differential output signal (absorption signal) as represented by thebar 67 is shown to be less under balanced conditions (FIG. than underthe extreme unbalanced conditions of the single beam instrument (FIG.5).

In FIG. 7, an intermediate degree of unbalance is shown to produce. anintermediate absorption signal represented by the bar 7l. Here, thenonattenuated sample beam has a flow waveform represented by the line72; and the unbalanced, smaller energy of the reference beam isrepresented by the line 73. Their difference, indicating the net pulsedflow through flow chamber 22 in the absence of the component of interestin the sample (no sample absorption), is shown by the line 74. When thesample contains the component of interest, however, the sample beam isattenuated to the form shown in line 76. The difference between theenergy represented by that line and the lesser energy of the referencebeam 73 is shown by the line 77. When transferred, the differencebetween the lines 76 and 73 produce the output signal 71.

Although maximum sensitivity of the instrument is obtained with maximumunbalance of the beams, the use of a reference beam is sometimesdesirable to compensate for ambient conditions or'in specialapplications-when a relationship by the absorption in the beams otherthan simple algebraic difference is desired. The unbalanced operationdescribed herein increases the sensitivity of the instrument,particularly for low absorption by the sample gas.

Weclaim:

l. In an infrared gas analyzer of the type in which the component ofinterest in a gas sample is'selectively detected and measured by itsabsorption of a pulsed beam of infrared energy,emitted by a source ofsuch energy, the improvement comprising: a sample cell adapted tocontain the gas sample to be analyzed and to be traversed by a pulsedbeam of infrared energy for attenuating said beam to the extent it isabsorbed by the component of interest in the gas sample, first andsecond gas chambers, the first chamber being disposed in the opticalpath of the beam leaving the sample cell and adapted to contain a gasabsorbing infrared energy in the same spectral region as the componentof interest for irradiation by said beam, the second chamber beingadapted to receive a fixed substantially different amount of infraredenergy, including no infrared energy at all, from that received by thefirst chamber in the absence of the component of interest in the samplecell a gas flow chamber, a separate passage connecting each gas chamberto the flow chamber and providing relatively high velocity gas flow intothe flow chamber as as flows from one chamber to the other in responseto the a ternate expansion and contraction of gas in at least one of thegas chambers when subjected to said pulsed energy, an electricallyheated resistance sensor responsive to cooling by gas flow and having arate of change of resistance that varies inversely with temperature, thesensor being mounted in the flow chamber in the path of the highveloc'ity gas flow from the passages to enhance the cooling effect ofsuch gas flow by the entrainment of cool gas in the flow chamber, and anelectrical circuit for measuring the change in resistance of the sensorin response to such gas flow in the absence of the component of interestin the sample cell and in the presence of such component therein.

2. Apparatus according to claim 1 that also includes a second pulsedbeam of infrared energy and a reference cell adapted to contain areference gas and to be traversed by the second pulsed beam of infraredenergy for attenuating said second beam to the extent that such energyis absorbed by the reference gas, the second gas chamber being disposedin the optical path of the second beam leaving the reference cell andbeing adapted to contain a gas absorbing infrared energy for irradiationby said beam.

3. Apparatus according to claim 2 that includes means for varying thedegree of unbalance between the infrared energies of the first andsecond beam.

4. Apparatus according to claim 3, in which the means for varying saidunbalance include a mechanical trimmer interposed in the path of thesecond beam.

5. Apparatus according to claim 1, in which the electrical circuitincludes means for neutralizing the effects of the changes in resistanceof the sensor caused by the unbalance in the pulsed beams in the absenceof the component of interest in the sample cell.

6. Apparatus in accordance with claim 1 in which the second gas chamberreceives no infrared energy.

2. Apparatus according to claim 1 that also includes A second pulsedbeam of infrared energy and a reference cell adapted to contain areference gas and to be traversed by the second pulsed beam of infraredenergy for attenuating said second beam to the extent that such energyis absorbed by the reference gas, the second gas chamber being disposedin the optical path of the second beam leaving the reference cell andbeing adapted to contain a gas absorbing infrared energy for irradiationby said beam.
 3. Apparatus according to claim 2 that includes means forvarying the degree of unbalance between the infrared energies of thefirst and second beam.
 4. Apparatus according to claim 3, in which themeans for varying said unbalance include a mechanical trimmer interposedin the path of the second beam.
 5. Apparatus according to claim 1, inwhich the electrical circuit includes means for neutralizing the effectsof the changes in resistance of the sensor caused by the unbalance inthe pulsed beams in the absence of the component of interest in thesample cell.
 6. Apparatus in accordance with claim 1 in which the secondgas chamber receives no infrared energy.